1. Field of the Invention. The present invention relates to a method for measuring crack propagation within a sample in order to monitor damage to structural components.
2. Description of Related Art. It is well documented that when structural materials are exposed to particular aggressive service environments, and where the material is under steady or cyclic stress, the material will be susceptible to damage in the form of cracking. This type of damage is commonly referred to by names such as "stress corrosion cracking" or "corrosion fatigue". Many industries must cope with the possibility that stress corrosion cracking may occur in operating equipment. The nuclear industry in particular continues to encounter this problem where the structural materials operate under sustained or cyclic stress in the presence of high temperature water, such as in boiling water reactors.
At least in some respects, damage in the form of stress corrosion cracking, or other stress/environment-induced cracking, hereinafter referred to collectively as stress corrosion cracking, is of much greater concern in industry than damage such as that caused by uniform corrosion, which results in a predictable service life for components, as material failures due to stress corrosion cracking are many times not easily predicted and are generally significant in nature.
U.S. Pat. No. 4,677,855 issued to Coffin, Jr. et al, the subject matter of which is hereby incorporated by reference, sets our problems which industry in general, and the nuclear industry in particular, faces in attempting to predict the onset of or susceptibility of particular structural components to stress corrosion cracking. In general, the performance of structural components is predicted in advance from information on the expected loadings and resulting stress from these loadings. Although these predictions are sufficiently accurate to predict service performance, it has been found difficult to predict the lifetime of such performance due to the uncertainty in the environmental conditions and the influence thereof on the stress corrosion cracking which results therefrom.
An example of the uncertainty of lifetime predictions for structural materials is the stress corrosion cracking which has been found to occur in stainless steel piping used in the nuclear industry. Although designs for new plants attempt to compensate for this phenomenon, it is desirable to monitor and assess the extent of damage in plants which have been operating for a number of years to help predict their lifetimes and possibly extend their lifetime. Methods for assessing the state of damage have been directed toward monitoring the aggressive environment. In the case of boiling water reactors, the water chemistry is measured to determine factors such as resistivity, electrochemical potential, oxygen level and impurity levels. Such measurements are indirect. No direct measurement is made of the effect this water chemistry has on crack growth in the structural materials during plant service. Therefore, the extent to which the lifetime of the structural material is extended by varying operating conditions are unknown.
Methods for directly measuring crack growth in specimens removed from their environment have been disclosed over the years. These methods use a variety of monitoring systems including visual and voltage potential drop methods, such as that disclosed by Beevers, Editor, "The Measurement of Crack Length and Shape During Fracture and Fatigue", Engineering Materials Advisory Services, Limited, (1980). However, it was not until the method disclosed in the Coffin, Jr., et al. patent, that the industry was provided with the capability to accurately assess crack growth of plant structural components through voltage potential drop methods by disclosing a reasonably accurate way to relate voltage measurements to crack size.